Solder application tools are well known in the prior art. In their least complex form, as illustrated in FIG. 1, the tool is comprised of a soldering tip (1) which is heated above the melting point of the solder. The soldering tip is held against, or near, the surface on which the solder is to be deposited (5). The solder, generally in wire form, (3) is placed against the soldering tip. The solder heats, and melts, running down the tip,and forming a meniscus (7). The meniscus is then brought in contact with the surface (5).
Somewhat more complex solder tools have heated reservoirs for the solder. These tools have nozzles through which the molten solder flows and which can be used to direct the solder deposit. An early form of such a soldering application tool can be found in U.S. Pat. No. 2,380,138, Abramson, "Solderer or Lead Tool." A somewhat later and more complex version is described in U.S. Pat. No. 4,934,309, Ledermann et al, "Solder Deposition System."
In applications such as the fabrication of computer modules, in which very precise soldering (on the order 0.002 inch wide pads on 0.004 inch centers) is required, it is critical that the solder tip of the tool be a specific distance from the pads (or surfaces to be coated) when the solder meniscus contacts and delivers solder to these pads. If the meniscus is too far from the surface to be soldered, the solder coating may not be even or too little solder may be deposited to ensure an adequate joint. If the meniscus is too close to the surface, too much solder may be deposited and it may flow in unwanted directions causing pad bridging.
In ordinary operation, as a natural result of gravity, each pad removes a finite amount of solder from the tip of the meniscus. This reduces the volume of solder at the point of contact, so that the solder meniscus will no longer be at the correct distance, will no longer contact the pads, and will not coat them sufficiently for adequate joins.
As a result, in prior art, considerable effort was expended in trying to compensate for the change in the volume of solder. Since part of the problem would be solved by providing an equal, constant amount of solder (essentially an "infinite" supply) during the soldering process, many of these efforts in the prior art concentrated on providing a reliable solder feed mechanism. Examples of this approach can be found in commercially available solder application tools such as the IBM Solderfoot.TM. (Trademark International Business Machines Corporation) or the Apollo Seiko Automatic Soldering Unit (Digital Apoauto Model). Generally, these tools use complex solder feed mechanisms and, as a result, are both costly and have a relatively large number of parts that are subject to malfunction.